In a large, nonreactive container, thoroughly mix cider vinegar, whole
grain mustard, garlic, lime juice, lemon juice, brown sugar, and salt. Whisk
in olive oil and pepper. Place chicken in the mixture. Cover, and marinate
chicken in the refrigerator 8 hours, or overnight. Preheat an outdoor grill
for high heat, and lightly oil grate. Place chicken on the prepared grill,
and cook 10 minutes per side, until no longer pink and juices run clear.
Discard remaining marinade.